A photonic network including either one or both of reconfigurable optical add/drop multiplexer (ROADM) and a wavelength cross connect has been suggested and developed. A reconfigurable optical add/drop multiplexer can split an optical signal having a desired wavelength from a WDM optical signal that includes a plurality of optical signals and guide the optical signal to a client, and can insert a client signal having any wavelength to the WDM optical signal. The wavelength cross connect (WXC: wavelength cross connect or PXC: photonic cross connect) can control a route of an optical signal for each wavelength without conversion to an electrical signal.
In the photonic network as described above, a plurality of optical paths (here, wavelength paths) using the same wavelength may be set. For this reason, for reliably constructing and operating the network, for example, a scheme of superposing a supervisory signal including information for identifying an optical path on an optical signal for transmission has been suggested. In this case, an optical node device (here, such as a reconfigurable optical add/drop multiplexer or a wavelength cross connect) on the photonic network includes a function of detecting the supervisory signal superposed on the optical signal. With this, in the optical node device, each optical path can be reliably identified, thereby making it possible to either or both of monitor and detect a failure such that an optical fiber is connected to an erroneous port.
The supervisory signal described above is superposed on the optical signal by frequency shift keying (FSK). Here, in the WDM transmission system, the supervisory signal can be superposed on an optical signal of each channel of the WDM optical signal.
FIG. 1 is a diagram for describing a method of detecting a supervisory signal superposed on an optical signal of each channel. In this example, channels of a WDM optical signal are arranged at predetermined interval. In the example depicted in FIG. 1, channels CH1, CH2, CH3, . . . are arranged at 50 GHz intervals.
An optical receiver has an optical filter letting part of a band of each channel pass through. In FIG. 1, the optical filter has passbands F1 to F4. The passband F1 lets part of the band of the channel CH1 pass through. Similarly, the passbands F2 to F4 let part of the bands of the channels CH2 to CH4, respectively, pass through. An output from this optical filter is converted by using an optical detector to an electrical signal, the supervisory signal superposed on the optical signal of each channel.
As such, by extracting part of the band of each channel by using an optical filter, the supervisory signal superposed on each optical signal is detected. Thus, if the channels of the WDM optical signal are arranged at predetermined intervals, by using an optical filter having periodic transmission characteristics with respect to the wavelength, the supervisory signals superposed on the plurality of optical signals can be simultaneously detected. In the example depicted in FIG. 1, an optical filter with transmission characteristics being changed at 50 GHz intervals is used.
Note that an optical monitor is suggested as related art (for example, Japanese Laid-open Patent Publication No. 2003-195097) in which the characteristics of a blazed Bragg grating are used to redirect part of an optical signal to a detection device. Still another related art is described in Japanese Laid-open Patent Publication No. 4-212111.
In a WDM transmission system in recent years, channels having different bit rates may be present in a mixed manner. Here, the bandwidth of the optical signal depends on the bit rate. That is, an optical signal with a high bit rate has a wide bandwidth, and an optical signal with a low bit rate has a narrow bandwidth.
FIG. 2A and FIG. 2B are diagrams for describing optical signals of different bit rates and their corresponding passbands. Note that the bit rate of the optical signal depicted in FIG. 2A is lower than the bit rate of the optical signal depicted in FIG. 2B. Therefore, the bandwidth of the optical signal depicted in FIG. 2A is narrower than the bandwidth of the optical signal depicted in FIG. 2B.
Here, to detect the supervisory signal superposed on the optical signal by frequency shift keying with high sensitivity, the passbands of the optical filter are preferably arranged in a region where a change in the spectrum of the corresponding optical signal is sharp. For this reason, if the bandwidth of the optical signal is narrow, as depicted in FIG. 2A, an offset frequency ΔF of the passband (a difference between a center frequency fc of the optical signal and a center frequency of a passband) with respect to the optical signal is decreased. On the other hand, if the bandwidth of the optical signal is wide, as depicted in FIG. 2B, the offset frequency ΔF of the passband with respect to the optical signal is increased. Thus, in the WDM transmission system where channels of different bit rates are present in a mixed manner, even if optical signals are arranged at predetermined intervals, the supervisory signal may not be detected with high sensitivity by using an optical filter having periodic passbands.
In an example depicted in FIG. 3, optical signals CH1, CH2, and CH4 have a bit rate of 100 Gbits/s, and an optical signal CH3 has a bit rate of 10 Gbits/s. Also, passbands F1 to F4 of an optical filter let part of the optical signals CH1 to CH4, respectively, pass through. Furthermore, the optical signals CH1 to CH4 are arranged at 50 GHz intervals. In this case, an interval between the passbands F1 and F2 is 50 GHz. However, an interval between the passbands F2 and F3 is narrower than 50 GHz. On the other hand, an interval between the passbands F3 and F4 is wider than 50 GHz. As such, when different bit rates are present in a mixed manner, to detect the supervisory signal superposed on each optical signal with high sensitivity, settings of the passbands of the optical filter are complicated.